Hydraulic fracturing of unconventional hydrocarbon reservoirs is critical to the United States energy portfolio; however, hydrocarbon production from newly fractured wells generally declines rapidly over the initial months of production. One possible reason for this decrease, especially over time scales of several months, is the mineralization and clogging of microfracture networks and pores proximal to propped fractures. One important but relatively unexplored class of reactions that could contribute to these problems is oxidation of Fe(II) derived from Fe(II)-bearing phases (primarily pyrite, siderite, and Fe(II) bound directly to organic matter) by the oxic fracture fluid and subsequent precipitation of Fe(III)-(oxy)hydroxides. The extent to which such reactions occur and their rates, mineral products, and physical locations within shale pore spaces are unknown. To develop a foundational understanding of potential impacts of shale iron chemistry on hydraulic stimulation, we reacted sand-sized (150-250 μm) and whole rock chips (cm-scale) of shales from four different formations

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Organic thiols are highly reactive ligands and play an important role in the speciation of several metals and organic pollutants in the environment. Although small thiols can be isolated and their concentrations can be estimated using chromatographic and derivatization techniques, estimating concentrations of thiols associated with biomacromolecules and humic substances has been difficult. Here we present a fluorescence-spectroscopy-based method for estimating thiol concentrations in biomacromolecules and cell membranes using one of the soluble bromobimanes, monobromo(trimethylammonio)bimane (qBBr). The fluorescence of this molecule increases significantly when it binds to a thiol. The change in the sample fluorescence due to thiols reacting with qBBr is used to determine thiol concentration in a sample. Using this method, small thiols such as cysteine and glutathione can be detected in clean solutions down to ~50 nM without their separation and prior concentration. Thiols associated with dissolved organic matter (DOM) can be detected down to low micromolar concentration, depending on the DOM background fluorescence. The charge on qBBr prevents its rapid diffusion across cell membranes, so qBBr is ideal for estimating thiol concentration at the cell membrane-water interface. This method was successfully used to determine the thiol concentration on the cell envelope of intact Bacillus subtilis to nanomolar concentration without any special sample preparation. Among the chemical species tested for potential interferences (other reduced sulfides methionine and cystine, carboxylate, salt (MgCl(2))), carboxylates significantly influenced the absolute fluorescence signal of the thiol-qBBr complex. However, this does not affect the detection of thiols in heterogeneous mixtures using the presented method.

Hexavalent chromium is a water-soluble pollutant, the mobility of which can be controlled by reduction of Cr(VI) to less soluble, environmentally benign Cr(III). Iron(II/III)-bearing clay minerals are widespread potential reductants of Cr(VI), but the kinetics and pathways of Cr(VI) reduction by such clay minerals are poorly understood. We reacted aqueous Cr(VI) with two abiotically reduced clay minerals: an Fe-poor montmorillonite and an Fe-rich nontronite. The effects of ionic strength, pH, total Fe content, and the fraction of reduced structural Fe(II) [Fe(II)/Fe(total)] were examined. The last variable had the largest effect on Cr(VI) reduction kinetics: for both clay minerals, the rate constant of Cr(VI) reduction varies by more than 3 orders of magnitude with Fe(II)/Fe(total) and is described by a linear free energy relationship. Under all conditions examined, Cr and Fe K-edge X-ray absorption near-edge structure spectra show that the main Cr-bearing product is a Cr(III)-hydroxide and that Fe remains in the clay structure after reacting with Cr(VI). This study helps to quantify our understanding of the kinetics of Cr(VI) reduction by Fe(II/III)-bearing clay minerals and may improve predictions of Cr(VI) behavior in subsurface environments.

The recent increase in unconventional oil and gas exploration and production has prompted a large amount of research on hydraulic fracturing, but the majority of chemical reactions between shale minerals and organic matter with fracturing fluids are not well understood. Organic matter, primarily in the form of kerogen, dominates the transport pathways for oil and gas; thus any alteration of kerogen (both physical and chemical properties) upon exposure to fracturing fluid may impact hydrocarbon extraction. In addition, kerogen is enriched in metals, making it a potential source of heavy metal contaminants to produced waters. In this study, we reacted two different kerogen isolates of contrasting type and maturity (derived from Green River and Marcellus shales) with a synthetic hydraulic fracturing fluid for 2 weeks in order to determine the effect of fracturing fluids on both shale organic matter and closely associated minerals. ATR-FTIR results show that the functional group compositions of the kerogen isolates were in fact altered, although by apparently different mechanisms. In particular, hydrophobic functional groups decreased in the Marcellus kerogen, which suggests the wettability of shale organic matter may be susceptible to alteration during hydraulic fracturing operations. About 1% of organic carbon in the more immature and Type I Green River kerogen isolate was solubilized when it was exposed to fracturing fluid, and the released organic compounds significantly impacted Fe oxidation. On the basis of the alteration observed in both kerogen isolates, it should not be assumed that kerogenic pores are chemically inert over the time frame of hydraulic fracturing operations. Shifts in functional group composition and loss of hydrophobicity have the potential to degrade transport and storage parameters such as wettability, which could alter hydrocarbon and fracturing fluid transport through shale. Additionally, reaction of Green River and Marcellus kerogen isolates with low pH solutions (full fracturing fluid, which contains hydrochloric acid, or pH 2 water) mobilized potential trace metal(loid) contaminants, primarily S, Fe, Co, Ni, Zn, and Pb. The source of trace metal(loid)s varied between the two kerogen isolates, with metals in the Marcellus shale largely sourced from pyrite impurities, whereas metals in the Green River shale were sourced from a combination of accessory minerals and kerogen.